Create a function that returns an array of all the integers between two given numbers start and _end.
range_of_num(2, 4) ➞ [3]
range_of_num(5, 9) ➞ [6, 7, 8]
range_of_num(2, 11) ➞ [3, 4, 5, 6, 7, 8, 9, 10]start will always be <= _end.start and _end are NOT included in the final array.start == _end, return an empty array.